M755LMRE Bios Problem

[davicito]

Well I have finally isolated the problem...THe voltage of 1.7 is not reaching the CPU ......when that voltage is present it runs but it eventually falls to .3V
and crashes...That voltage should be present on the rectifier MOSPEC S20S40C.....the question is what causes this ? Is it the regulator chip or ???? I do know it´s not the Power Supply, since I´ve used at least 4 different ones..

The big question is how do I fix it ????

At least it´s been a learning experience !!

[Platypus]

You're doing a sterling job of fault-finding.

Although I do component-level repairs, Ruslan is our member with the most direct experience in this area. If he doesn't notice this thread of his own accord, perhaps you could attract his attention with a private message.

It is still possible that electrolytic capacitors could be contributing to the problem, if they are developing high series impedance, even if they have not begun to visibly leak or bulge. It may be more likely that a switching mosfet transistor is going bad, PCChips is known for being weak there too.

[davicito]

Thank you for your motivation!

Without removing the caps how can I tell if one is goin bad ?

[Platypus]

For that kind of fault an ESR (Equivalent Series Resistance) Meter can reveal the deterioration, and may be effective "in-circuit". Where capacitors are in parallel, as they often are on supply lines, it's best to be able to test each individual component. In the absence of test equipment, but if you have suitable soldering facility, for the cost of a few high ripple-current rated capacitors, substitution is the simplest action.

If you have ready access to the reverse of the board it could also be worth checking for evidence of bad solder joints around the CPU core voltage conversion components.

[Ruslan]

It could be actually bad capacitors... Take a closer look again - especially at those 7 caps located near power connector as they are the ones responsible for stable CPU voltage. By the way, on a picture given by Garak, were 8 defective caps (out of 12), did you spot them all?
Any capacitor with top other than ideally flat is defective! Also, capacitors could be damaged on pins (bottom) side, not only tops... Check it out too...

I wouldn't trust much an ATX power supply (especially the one what came with a cheap motherboard like this one) as PC builders usually use the cheapest PSU/Case combo and cheap PCChip/ECS MB to build the cheapest PC...
You can open that PSU and check if there is no any burn signs and/or defective caps inside... AT PSUs were much more reliable, IMHO.

Unfortunately, bad power supply could destroy some of motherboard's components - fisrt of all, CPU voltage regulator. And looks like you have problems exactly with that. If all the capacitors are fine, check the power MOSFET (Field-Effect Transistor) located near that MOSPEC chip...
Measure Case-to-pins resistance at lowest range - shouldn't be Zero in any direction unless that MOSFET is fully opened. It is possible to open it by applying about 5-10V to the Gate-Case pins (some of multimeters can do it, some - not)... Apllying reverse polarity voltage to the same pins should close the MOSFET then. It that MOSFET if good (and I'm pretty sure it would be so), then voltage controller chip is the one to be blamed. Most of an older PCChips motherboard are using 34063A voltage controller chips, one (at least) or two. This motherboard has 34063 chip soldered (see picture below)
close to AT power connector... Locate it, download the PDF file for typical schematics... If you have an oscilloscope, you can check the signal coming out of that chip to the gate of MOSFET... May be this chip needs to be replaced - you can find it in local electronic shop (cost around 30-50 cents) or desolder it from another PCChips board...

There were at least two different revision of that board, by the way http://www.pcchipsusa.com/prod-m755mr.asp - so make sure you have a proper BIOS version...

[Garak]

By the way, on a picture given by Garak, were 8 defective caps (out of 12), did you spot them all?
.

I got em. Thanks for the additional pointers

[davicito]

I have definitely double checked all the caps and they are ok...My board is definetely V7.1 the one depicted on picture to the right. I downloaded the PDF for the 34063A although mine reads 34063P...I´ll follow your tips and let you know...Thank you

I can see the 8 bad Caps on Garaks pic !!

[davicito]

To all who have answered:

This link will show the mobo I have : It´s the one in the right aV7.1:

http://www.pcchipsusa.com/prod-m755mr.asp (sent by Ruslan)

Here is the latest. I have noticed that the negative -5V supplied by the PS
falls to -4.7 or -4.8 as soon as the board dies. I have tried several PS´s and the problem persists. I gather the board has an overconsumption problem somewhere that causes the problem, since all power supplies show same problem. When the board dies the video goes with it. I have even used 2 PSU´s at the same time, one for the MoBO and another for the peripherals!

This board has 2 controllers 34063P..one next to the AT connector and another next to the BIOS. When the voltage drop occurs both of these have bad voltages also. While the board is working they both show the correct voltage. Th one next to the connector has 4.7 in pin 2( Switch emitter) and the one by the BIOS reads 7.5V.

The problem is I´d hate to desolder these guys and overheat the board and cause extra damage. I have a crazy idea that woulbe to piggy back a new one on top and cut the Vcc pin from the one on the board. Will this work ????